UNIT IV- ENGINEERING MATERIALS-II

Q1. What is a semiconductor? What are intrinsic and extrinsic semiconductors?

A semiconductor is a substance, usually a solid chemical element or compound that can

conduct electricity under some conditions but not others, making it a good medium for the

control of electrical current.

It has almost filled valence band, empty conduction band and very narrow energy gap i.e.,

of the order of 1 eV. Energy gap of Silicon (Si) and Germanium (Ge) are 1.0 and 0.7 eV

respectively. Consequently Si and Ge are semiconductors.

Effect of temperature on conductivity of semiconductors: At 0 K, all semiconductors are

insulators. At finite temperature, the electrical conductivity of a semiconductor material

increases with increasing temperature. With

increase in temperature, outermost electrons

acquire energy and hence by acquiring energy,

the outermost electrons leave the shell of the

atom.

Hence with increase in temperature, number

of carriers in the semiconductor material

increases and which leads to increase in

conductivity of the material.

Types of semiconductors:

Intrinsic Semiconductor: An intrinsic semiconductor material is chemically very pure and

possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive

carriers (holes). A silicon crystal is different from an insulator because at any temperature

above absolute zero temperature, there is a finite probability that an electron in the lattice will

be knocked loose from its position, leaving behind an electron deficiency called a "hole".

This hole can travel from one atom to the

adjacent atom by accepting an electron from later

one.

This process involves formation of new

covalent bond and breaking an existing bond by

filling up the hole and creating a new hole.in this

way, the holes travel fromone atom to the adjacent

atoms in crystal lattice.

If a voltage is applied, then both the electron and the hole can contribute to a small current

flow.

Extrinsic Semiconductor: Extrinsic semiconductor is an improved intrinsic semiconductor

with a small amount of impurities added by a process, known as doping, which alters the

electrical properties of the semiconductor and improves its conductivity. Introducing

impurities into the semiconductor materials (doping process) can control their conductivity.

Doping process produces two groups of semiconductors: the negative charge conductor

(n-type) and the positive charge conductor (p-type). Semiconductors are available as either

elements or compounds. Silicon and Germanium are the most common elemental

semiconductors. Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC,

GaN.

Si and Ge both have a crystalline structure called the diamond lattice. That is, each atom

has its four nearest neighbours at the corners of a regular tetrahedron with the atom itself

being at the centre. In addition to the pure element semiconductors, many alloys and

compounds are semiconductors. The advantage of compound semiconductor is that they

provide the device engineer with a wide range of energy gaps and mobilities, so that materials

are available with properties that meet specific requirements. Some of these semiconductors

are therefore called wide band gap semiconductors.

Q2. How doping of semiconductors is done?

The Doping of Semiconductors

The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon

or germanium produces dramatic changes in their electrical properties, producing n-type and

p-type semiconductors.

Pentavalent impurities

(5 valence electrons) produce n-type semiconductors by contributing extra electrons.

Trivalent impurities

(3 valence electrons) produce p-type semiconductors by producing a "hole" or electron

deficiency.

N-Type Semiconductor:

The addition of pentavalent impurities such as

antimony, arsenic or phosphorous contributes free

electrons, greatly increasing the conductivity of

the intrinsic semiconductor. Phosphorous may be

added by diffusion of phosphine gas (PH3).

P-Type Semiconductor

The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic

semiconductor creates deficiencies of valence electrons, called "holes". It is typical to use

B2H6 diborane gas to diffuse boron into the silicon material.

Note: positive holes are less mobile than free electrons so the conductivity of a P-type

semiconductor is inherently less than that of N-type semiconductor.

Q3. What are the differences between p-type and n-type semiconductors?

Differences between p-type and n-type semiconductors:

N-type Semiconductor P-type Semiconductor

It is an extrinsic semiconductor which is

obtained by doping the impurity atoms such

as antimony, phosphorus, arsenic, etc. to the

pure germanium or silicon semiconductor

It is an extrinsic semiconductor which is

obtained by doping trivalent impurity atoms

such as boron, gallium, indium etc.to the pure

germanium or silicon semiconductor.

The impurity atoms added, provide extra

electrons in the structure, and are called

donor atoms.

The impurity atoms added, create vacancies of

electrons (i.e. holes) in the structure and are

called acceptor atoms.

The electrons are majority charge carriers

and holes are minority charge carriers.

The holes are majority charge carriers and

electrons are minority carriers.

Q4. How semiconductors are prepared by different techniques? Describe in detail.

Preparation of Semiconductors: For preparing semiconducting devices, the basic

materials such as Si and Ge must at least 99.9999% pure. Consequently extreme precautions

have to be taken during the preparation of these elemental semiconductor materials. Various

steps followed during the preparation of highly pure materials are as follows:

Preparation of pure Ge/Si: This is carried by using the following methods:

1. Distillation: is based on tge separation of materials due to difference in boiling points. The

starting raw materials employed are germanium tetrachloride and trichloro silane for the

preparation of pure Ge and Si respectively.

For example GeCl4 (b.p. 83.10C) is taken in a series of distillation stills and a layer of HCl

placed over the charge, which facilitates in the removal of As (usually present as an

impurity). Chlorine gas is then passed through the charge, which is heated electrically. The

vapours passing through the fractionating column finally distils over in a receiver, kept in an

ice bath. Pure GeCl4 so obtained is then treated with extra pure water to get germanium oxide

which is subsequently reduced (in an atmosphere of hydrogen) to get elemental Ge, which is

subjected to further purification. Similarly, distilled trichloro silane (SiHCl3) is employed to

get silicon of high purity.

2. Zone refining:

The zone refining technique involves melting a narrow zone of long ingot of the material

to be processed. This molten zone is then slowly moved by moving the heater gradually along

the length of the ingot in one direction. This process is repeated multiple times till the

required purity level is reached. In this process difference between solubility of impurities in

the molten and solid phases is used to concentrate impurities at the end section of the ingots.

This method is particularly used when metals are required in high degree of purity. It is

based on the principle that when an impure metal in a molten state is allowed to cool, only

the metal crystallizes while the impurities remain present in the molten state (mass) or melt.

The impure metal converted into a rod which is heated at one end with a circular heater. A

narrow zone of metal is melted. The heater is slowly moved along the rod. The pure

metal recrystallizes out of melt while the impurities remain in the melt which moves along

with the melted zone of the rod with the movement of heater. The process is repeated several

times. The end of rod where impurities have collected is cut off. This method is employed for

the purification of germanium, silicon, gallium etc, which are used in semi – conductors.

3. Czochralski Crystal pulling Technique:

Ge or Si obtained by zone refining method is polycrystalline, i.e., there is no regularity in

their crystal structure and contain crystals of different sizes. But fabrication of most of the

semiconductor devices requires single crystal of semi conducting materials. Consequently,

they are grown in the form of single crystals. This method involves growing the crystal on a

single crystal seed; thereby the atoms reproduce identical atomic arrangement as that of the

seed crystal. In this process, solid polycrystalline semiconducting material is taken in a quartz

chamber and heated by using RF power source, till it melts. Then the temperature is lowered

to just above the melting point of Ge. At this stage, properly oriented single crystal of Ge,

attached to a puller rod is carefully lowered into the melt, till it is in contact with the surface

of the charge. Then the crystal is rotated slowly and then gradually pulled out so that the melt

in contact with the seed crystal starts solidifying producing the orientation identical as that of

the seed crystal. After this, the pull rate is kept at about 1.5 to 5 cm/minute and the rotation

speed is maintained at about 100rpm.Finally, an intrinsic crystal of Ge of 5cm diameter and

25 cm length is obtained. This crystal is tested for its conductivity and cut into wafers with

the help of a diamond-tipped saw.

Doping techniques: For getting a doped material, calculated quantity of dopant (Boron or

Phosphorus) is added to the melt before crystal pulling stage. Usually dopant atoms added are

of the order of one atom of B or P per 108 atoms of Ge or Si. Other methods of doping are:

Epitaxy: Involves in unified crystal growth or deposition of a thin crystal on another

substrate, often with added dopants. The material in the epitaxial layer must have a little

spacing and structure closed to that of the substrate. ex. Si on Ge or Ge on Si. The Si or Ge

wafer (kept in graphite boat) is placed in a long cylindrical quartz tube reactor, which is then

heated (by RF induction coil). Then gases containing compounds of Ge or Si mixed with

calculated/appropriate quantities of dopant over the wafer results. The doped wafer so

obtained is ready for the fabrication of semiconductor devices. It may be pointed out that for

getting Si epitaxial film, SiCl4 H2 and N2 mixture is used. For carrying out n-type doping, this

mixture is mixed with a calculated amount of phosphine (PH3) and for p-type doping;

Diborane (B2H6) is employed.

Diffusion technique: Involves in conversion of a region of semiconductor material by solid

or gaseous diffusion of impurity atom into the crystal lattice of the semiconductor material

without any melting and it consists of heating a P-type dopant (Phosphorus or Stilbium). This

causes some of the impurity atoms to condense on the surface of water and diffuse thereby

resulting in a P-type semiconductor material.

By this technique, the extent of impurity atoms penetration can be controlled even less

than a few millions of a centimetre by controlling the concentration of dopant atoms and

temperature. This technique is especially employed for preparing n or p type semiconductors,

used in very high frequency devices. This doping technique can also be used to prepare n-p-n

type transistor with a base width of even 5 microns.

Ion implantation technique: Involves in bombarding the semiconductor material with an

electrically controlled beam of high energy ions of B or P of 10000ev. This causes the

implantation of some dopant atoms into the crystal lattice of semiconductor.

Main advantages of this technique are: 1. Implantation at low temperature 2. Extremely

precise control of the dopant amount on desired specific semiconducting material 3.

Obtaining deep or shallow regions of very precise and required penetration depths having

high or low concentration of dopant. This technique is especially employed for the fabrication

of high frequency devices.

Cutting into chips: the wafers obtained by above methods are then cut into chips by using

methods such as chemical etching, ultrasonic vibrations, laser beams etc.

POLYMERS

Introduction:

Polymers are giant molecules produced by the combination of smaller units(polymer is

derived from Greek word(poly = many) and (mer = parts or units).

Hence polymer is giant molecule of high Molecular weight built up by linking together of

a large no. of smaller molecules called monomers.

Nature is the first scientist to produce the polymers in its “giant laboratory”.

Ex: Evolution of earth by combination of elements like C, H, O & N.

• Protein is the polymer – synthesized by nature from methane, ethene, ammonia & CO2

• Some natural polymers are wood, cotton, cellulose.

• Synthetic materials existing in form of polymer are plastics, fibers, gums, laminates, paints

etc…

Some polymers have gone to the extent of replacing ailing hearts and other ones sensitive

organs.

Polymers possess the desirable properties like high strength light wt., good flexibility,

special electrical properties resistance chemicals and fabrication into complex shapes in a

wide variety of colors.

Naturally occurring macromolecules are cellulose; wood and rubber are very widely used in

day today life.

Concept of polymerization:

The polymer (i.e. macromolecules) is the resulting structure comprises of repetition of

relatively small molecular fragments referred to as repeating units.

Ex: polythene is made up of interlinking of ethylene

Ethylene monomer polyethylene repeating unit

Depending upon degree of polymerization there are 2 types, Degree of polymerization of

polythene is 100

• Oligo polymers: - Polymers whose degree of polymerization is less than 600 are called

oligo polymers.

These do not possess engg. properties.

• High Polymers: - Degree of polymerization of polymer is more than 600 they possess

desired engg. properties.

Functionality of Monomers:-

For a substance to act as a monomer, it must have at least two reactive sites or bonding

sites. No of reactive or bonding sites in a monomer is called functionality.

If x reactive sites are there in a monomer is bi functional – they form linear or straight

chain molecules

Q1. What are the types of polymerization? Explain with examples.

Addition (or) chain Polymerization: -

It is a reaction that yields a product which is an exact multiple of the original monomeric

molecule.

The monomer contains one or more double bonds and it is bi functional.

It must be initiated by light, Heat, Pressure (or) a catalyst for breaking down double

covalent bonds of a monomer.

Ex: Olefins, Vinyl compounds, Allyl and dienes.

Olefins:-

• Vinylcompounds:-

• Allyl compounds:-

• Diene compounds:-

II. Condensation (or) step polymerisation: -

It involves the reaction occuring b/w polar gps containing monomers with formation of

polymer and elimination of small molecules like water, etc.

Ex: Nylon 6,6 : -Hexamethylene diamine and adipic acid condense to from nylon 6:6

The types of functional groups, which are most important in condensation reactions, are

--OH] + --COOH[ -- OOC -- with elimination of H2O

Ex: polyethylene terephthalate(terylene)

--NH2 + -- C O O H -- NH --CO-- + H2O

Polyamide

Ex: spandex fibre

III copolymerization : - (it is also addition polymerization high M.wt compounds obtained

by copolymerization, are called copolymers.

2 diff. monomer units combine to form polymer

Polymerization like in addition and it requires special catalyst)

Co – ordination Polymerisation (or) Ziegler – Natta polymerization

Ziegler & Natta proposed that presence of combination of transition metal halides like

TiCl4, ZnBr3… with an organometallic compound like (C2H5)3Al (or) (CH3)3Al

polymerization can be carried out. It explains the stereo specific type of polymerization.

Q2. What is a plastic? Explain the differences between thermoset and thermoplastic

materials. Give examples.

Plastics are polymers characterized by property of plasticity (permanent deformation in

structure on applying some stress / force) and moulded to desired shape upon subjected to

heat and press in presence of catalyst.

Resin : - It’s the product of polymerization and form the major part of plastics.

Thermoplastic Resin Thermoset Resins

1 They soften on heating readily They do not soften on heating on

prolong heating, they burn

2 Consists of long chain linear

molecules

They have 3 –d network joined by

strong covalent bonds

3 Formed by Addition polymerization Condensation – polymerization

4 They are soft, weak & less brittle They are hard, strong and more brittle

5 They can be reclaimed from wastes Can’t be reclaimed from wastes

6 Usually soluble in some org. solvents Due to strong bonds & cross liking they

are insoluble in all org. solvents

Q3. How compounding of plastics is done?

Compounding of plastics: - Polymer Resin is mixed with 4 to 10ingredients during

fabrication to impart useful properties to the finished article, this is called compounding of

plastics

• Resin: - it’s a binder which holds the diff. constituents together. Thermoset resins are

supplied as linear polymers of comparatively low M.wt. This is converted to cross

link infusible from during moulding.

• That is added to resins to increase their plasticity and flexibility.

• They neutralize the intermolecular forces of attraction b/w polymer chains.

• They impart greater freedom of movement b/w the polymeric molecules

Plasticizers decrease the strength and chemical resistance. Commonly used plasticizers are

vegetable oils, camphor, esters of stearic acid … tricresyl phosphate and trivinyl phosphate.

3. Fillers: -

• They are added to give final plastic better hardness, tensile strength, opacity and

brittleness.

• They reduce cost of polymers

• Some special type of fillers are added to impart special properties to polymers like Ba

salts make polymers impermeable to x-rays and Asbestos provides heat & Corrosion

resistance to polymers

Percentage of fillers is up to 50% of total moulding mix commonly used fillers are wood

flour, gypsum saw dust, marble flour, paper pulp, cotton fibres and metallic oxides like

Zno & Pbo.

4. Lubricants: - a. waxes oils stearates and soaps are employed to make the moulding of

plastic easier

b. They prevent plastic material from sticking to the fabrication equipment.

5. Calalysts or Accelerators:-

They accelerate the polymerization of fusible resin during molding operation into cross

linked infusible form for thermosetting resins.

Commonly used catalysts for compounding of thermosetting resins are benzoyl peroxide,

H2O2 acetyl H2SO4, metals like Ag, Cu & lead, metal oxide like Zno.

6. Stabilizers: -

They improve thermal stability during polymerization.

• During moulding heat stabilizers are used

Ex: opaque moulding compounds like salts of lead, lead chromate, litharge, red lead,

lead silicate and lead naphthalene.

Transparent moulding compounds like stearates of lead, Cd and Ba.

7. Colouring Materials: -

• Colouring is of high importance in high polymer articles.

• Colouring materials impart esthetic sense and appeal to plastics.

• Organic dyestuffs and opaque inorganic pigments are commonly used as colouring

materials.

Summary of compounding of plastics

• Resin forms body of the plastics and binder for diff ingredients.

Ex: Thermoplastic & Thermoset resin.

• Plasticizer gives plasticity and flexibility.

Ex: Oils, water, tricresyl and triamyl compounds.

• Fillers | reinforced fillers give hardness, tensile strength, opacity, finish workability to

plastics, reduces the cost.

Ex: Ba salts, quartz, mica, ZnO, PbO, Al, Cu, Pb.

• Lubricants makes moulding easier fives flawless gloss finish to plastics.

Ex: waxes, oils, stearates and oleates.

• Catalysts | accelerators accelerates the cross linking of thermosetting resin.

Ex: H2O2, Cu, Ag, Pb.

• Stabilizers increase thermal stability during fabrication

Ex: Pb salts, Ca & Ba

• Colouring materials gives esthetic sense and appeal to plastics.

Ex: organic dyes & opaque inorganic pigments

Moulding of plastics into articles

To give suitable shape to plastic several methods of fabrications are in used.

The method chosen depend upon type of plastics (i.e. thermo or thermosetting plastics)

and shape of finished product commonly used methods are given below.

Q4. Explain different types of moulding techniques involved in preparation of plastics.

Compression Moulding: -

This is applicable to both thermoplastic and thermosetting resin. A known quantity of

compounded plastic resin is filled in the cavity present in the cavity present in the bottom

mould. Top mould and bottom mould are capable of being moved relative to each other when

heat and press are applied according to specifications the cavities get filled with fluidized

plastic.

The two mould (top & bottom in fig.) are closed tightly and curing (time given to polymer

to set in the mould) is clone either by heating in case of thermoplastic resins or by cooling in

case of thermoplastic

resins

After curing the

moulded article is

taken out by opening

the mould.

Uses: the process is

used for manufacturing

of cylinders and gear

handles.

Injection moulding: - (Applicable to thermoplastic resins)

The moulding plastic powder is fed into heated cylinder from a hopper is fed into heated

cylinder from where it is injected at a controlled rate into tightly locked mould by means of

screw arrangement or by piston the mould is kept cold to allow hot plastic to cure and

become rigid.

When material have been cured sufficiently half of the mould is opened to allow the

injection of finished article without any deformation. Heating is done by oil or electricity.

Advantages : - Mainly useful for moulding of thermo plastics because of high speed

production, low mould cost, very low cost of material and low finished cost.

Limitations: - large no. of cavities can’t be filled simultaneously. So there is limitation of

design articles to be moulded.

Q5. How is HDPE prepared? Write its properties and applications.

High density polyethylene: density is 0.965 g/cc.

It is prepared by using coordination chain polymerization mechanism.

Production of HDPE by coordination polymerization requires:

o temperature 50-75oC

o slight pressure

o a coordination catalyst is prepared as a colloidal suspension by reacting an aluminium

alkyl and titanium chloride (TiCl4) in a solvent such as heptane (C7H16).

o The polymer (polythene) forms as a powder or granules which are insoluble in the

reaction mixture. When the polymerization is completed, the catalyst is destroyed by

adding water or alcohol to the reaction mixture. The polymer (polythene) is then filtered

or centrifuged off, washed and dried.

Properties & uses: -

• The HDPE molecules are linear and their packing is easy. Hence HDPE has high density

(0.95-0.97) and more percentage of crystallinity (80-90%). Softening temperature of

HDPE is also high.

• It has excellent chemical resistance and electrical insulation property. It is stiffer, harder

and possesses greater tensile strength. Polyethylene is a rigid, waxy white, translucent,

non – polar material with high symmetrical structure.

• Polyethylene is a good electrical insulator.

• Resistant to strong acids, alkalis and salt solution at room temp.

• Its soft flexible polymer

• Resistant to atmospheric conditions like O2, CO2, moisture.

• HDPE has MP 144 - 150oC it has higher tensile strength and hardness with less gas

permeability

• LDPE possess branched chain structure and its M.P is 81oC

Engineering Applications:-

Polyethylene used for making high frequency insulator parts, bottle caps, packing

materials, tubes, coated wires, in chemical plants for kitchen and domestic appliances.

Q6. Mention the properties and uses of PVC.

Poly vinyl chloride:-

Vinyl chloride is prepared by treating acetylene with HCl at 60-80o

In the presence of metal oxide catalyst,

Properties: PVC is colourless, odourless, non-inflammable and chemically inert powder.

Pure resin possesses greater stiffness and rigidity compared to poly ethylene. It is brittle in

nature.

Engineering applications:-

1. Used for electrical insulations.

2. Injection moulding articles like tool handles ,radio and telephone components.

• Used for making safety helmets, refrigerator components,tyres

• And motor cycle mudguards.

Q7. Explain the preparation properties and uses of Bakelite.

Bakelite:-

(Phenol-formaldehyde resin). This belongs to thermoset resin.

Phenol reacts with formaldehyde in presence of acid or alkali produces mono di and

triMethylol phenols depending on phenol formaldehyde ratio.

Properties: phenolic resins set to rigid, hard, scratch resistant, water resistant, insoluble

solids, which are resistant to non-oxidizing acids and many organic solvents.

Engineering applications:-

a. for making electric insulator parts like switches, plugs, switch boards etc…

b. for moulding articles like telephone parts,cabinets for radio and television.

c. as adhesives for grinding wheels.

d. for making bearings,used in propeller shafts for paper industry and rolling mills.

Q8. Explain the preparation properties and uses of Nylon-6,6.

Nylon-6,6 (polyhexamethylene diamine adipamide) is a polyamide made from adipic acid

and hexamethylenediamine by polycondensation. The nylon developed by Carothers at Du

Pont was nylon 66. Because of the importance of starting out with equal amounts of the two

reactants, salts of the diamine and of the di-acid are made and then used in the commercial

synthesis of nylon 66.

Properties:

• Nylons possess high strength, hardness and high melting point. The high melting point of

the nylons is due to the hydrogen bonding between the hydrogen atom of the amide group

(-NH) and oxygen the С = О group.

Hydrogen binding in Nylons

• Nylons possess high crystallinity, abrasion resistance and good mechanical properties

• Nylons possess good chemical resistance,

• They are good electrical insulators.

• Nylons possess good resistance to environmental conditions.

Engineering applications:

• The major application is in the textile industry.

• Because of its high thermal and abrasion resistance nylons are used in mechanical

engineering applications like gears, bearings, machine parts where greater friction is

there

• Flexible tubings for conveying petrol etc are made from nylons.

• Nylons are used as electrical insulators.

• Nylon 6 is used for making tire cords.

• Nylons are used in auto mobile industry and telecommunication industry for making

radiator parts and coil formers respectively.

Q9. Explain properties and applications of butyl rubber.

Butyl rubber: also known as GR-I rubber.

Produced by copolymerisation of isobutene + 1-5% of butadiene (anhydrous aluminium

chloride in methyl chloride)

Properties:

o Strong, tough, low permeability to air and other gases.

o Excellent resistance to heat, abrasion, ageing, and chemicals such as inorganic

solvents, polar solvents

o Soluble in hydrocarbons solvents like benzene

o Abrasion resistant

o High resistant to atmospheric gases(ozone)

o Good electrical insulator .

Applications:

Cycle tyres, automobile parts and its tubes

Conveyor belt in food processing

Insulator for high voltage wires and cables

Tank lining

Q10. Explain properties and applications of Thiokol rubber. (Thiokol rubber or

polysulphide rubber or grp rubber):

Preparation: by copolymerisation of sodium polysulphide and ethylene dichloride.

Properties:

o Strength, impermeable to gases

o Cannot form hard rubber

o Good resistance to mineral oils, fuels, oxygen, solvents, ozone and sunlight

o Low abrasion resistance

Applications:

o Fabric coated with thiokol used for barrage balloons, lift rafts and jackets which

are inflated by carbon dioxide.

o Lining hoses for conveying gasoline and oil

o Making gaskets and seals for printing rolls

Q11. Explain the preparation, properties and uses of SBR rubber.

(Styrene butadiene rubber or SBR or Ameripol):

This is copolymer of 75% butadiene and 25% styrene. These are mixed in reaction mixture

containing an aqueous solution of an emulsifying agent. Initiator like cumene hydroperoxide

and p-methane hydroperoxide in presence of anti-freeze components to produce cold sbr or

cold rubber.

Properties: vulcanisation is similar to natural rubber but less sulphur is required, c-black is

added as a filler to improve physical properties.

Uses: light duty tyres, belts, gum, floorings, rubber shoe soles, and for electrical insulation.

Q12. Write a short note on liquid crystal polymers.

Liquid crystals are materials that behave in some ways like solids and in some ways like

liquids. The first known polymer liquid crystal solution was invented by Kwolek from a

polyamide known as Kevlar .Its repeat unit had this following structure which forms very

strong fibres.

Liquid crystallinity in polymers may occur either by dissolving a polymer in a solvent

(lyotropic liquid-crystal polymers) or by heating a polymer above its glass or melting

transition point (thermotropic liquid-crystal polymers). Liquid-crystal polymers are present in

melted/liquid or solid form. In solid form the main example of lyotropic. LCPs is the

commercial aramid known as Kevlar.

kevlar

Vectra

Properties of liquid crystal polymers:

• These polymers are capable of forming regions of highly ordered structure while in

liquid phase. The degree of order is somewhat less than that of a regular solid crystal.

• The liquid crystal polymers have high mechanical strength at high temperatures.

• These liquid crystal polymers possess extreme chemical resistance.

• They possess inherent flame retardancy and good weatherability.

• They can be easily fabricated into a variety of forms.

• LCP can be welded. The lines created by welding are the weak points in the resulting

product.

• LCP has high z- axis coefficient of thermal expansion.

• LCP resist stress cracking the presence of most chemicals at elevated temperatures.

Applications of LCP:

LCP are sometimes called ‘super polymers’. Their wide range of exceptional properties

and case of processing make them design for many demanding applications.

• LCP thermoplastic fibres possess exceptional strength and rigidity, suitable for

industrial, electronic and also space applications as well as high performance ropes and

tennis rackets.

• LCP finds extensive applications as coatings, composites and additives.

• The electrical motor components are made from LCP.

• LCP finds its applications in electronic industry as LED’s and SMT components.

• LCP has an interesting application like information storage media.

Q13. Write a short note on biodegradable polymers.

Biodegradable polymers: polymers are not attacked by environmental conditions

including biological attack. Polymers are degraded by oxidation, u.v. radiation etc. but not by

bacteria.

Biodegradable polymers are defined as degradable polymer in which degradation results

from the nature of naturally occurring microorganisms such as bacteria fungi and algae

The biodegradable polymers may be naturally occurring or they may be synthesized to

chemicals.

Naturally occurring biodegradable polymers : they are classified into four groups.

Polysaccharides proteins polyesters others

Ex: Starch and Ex: Gelatin, Casein Ex: polyhydroxy alkanoils. Ex: lignin, shellac,

Cellulose silk, Wool. Natural rubber.

Natural polymers are for environment degradation. The rate of degradation depends upon

the structural complexity of material and environmental conditions.

Synthesised Biodegradable polymers: polymers derived from petrochemicals or biological

sources are biodegradable. They are used in dissolving suture material in medical field, and

bio-polyesters. Because of their high prices new type of polymers were developed of

chemicals derived from farming of particular crops, which laid foundation to new synthetic

biodegradable polymers.

Exs: 1. Polyalkaline esters, 2. Polylacticacid and its copolymers, 3.Polyamide esters, 4.

polyvinyl esters, 5. Polyvinyl alcohol, 6.poly anhydrides

All the above biodegradable polymers possess particular properties and potential

applications.

Applications: 1. biodegradable polymers are synthesized from the processing of crops or

from petrochemical feed stock with normal or conventional processing methods.

• The compostable bags help in the disposal of vegetable matter being converted to CO2

and CH4.

• The problem of landfills by solid waste can be reduced.

Drawbacks: They are more expensive than the products manufactured from commodity

resins.

Q14. What are conducting polymers. Write their applications and uses.

Conducting polymers: Those polymers which conduct electricity are called conducting

polymers. The conduction of the polymers may be due to unsaturation or due to the presence

of externally added ingredients in them. The conducting polymers can be classified in the

following way.

Conducting polymers

Intrinsic Conducting polymers Extrinsic Conducting polymers

(Conjugation due to presence of Double bond) (Due to externally added reagents)

Intrinsic polymer

with conjugation

Doped

Conducting

polymers

Conductivity

element filled

polymer

Blended

Conducting

polymers

Intrinsic Conducting polymers: Due to presence of intrinsic conjugation of double

bonds in their structure. It is classified into two types.

1. Conducting polymers having conjugation: polymers having conjugative double

bonds in backbone possess due to pi electrons.

2. Doped Conducting polymers: Conducting polymers having II electrons can

easily oxidize or reduce because of low ionization potential and high electron

affinities. This can be done by 2 ways.

1. p-doping (creating positive site on polymer backbone):-

It can be done by oxidation of conducting polymer like poly acetylene with Lewis acid or iodine

vapour. Also called as oxidative doping.

During oxidative doping removal of II electron from polymer backbone lead to formation of

delocalized radical ion called as polar ion.

Extrinsic:

Applications:-

• The conducting polymers are used in rechargeable batteries.

• Used for making analytical sensors for pH, O2, SO2, NH3 and glucose.

• Used for electron beam lithography.

• They are applicable in photo voltaic cells.

Advantages:-

o Polymers possess good conductivity.

o They can store charge.

o They absorb visible light to give coloured complex.

Q15. Write a short notes on OLED’s.

Organic light emitting diodes generate light from electrical charges by exploiting the

process of electro-luminescence or electro-phosphorescence. An organic light emitting diode

(OLED) is basically a light emitting diode (LED). A suitable organic compound (organic

semiconductor) which emits the light in response to an electric current is situated between

two electrodes; Typically, at least one of these electrodes is transparent for transmitting the

emitted light. By applying potential difference between anode and cathode, holes and

electrons are injected respectively in to the organic layer, which emits the light on

recombination.

Figure 1: Schematic representation of an OLED

The energy diagram (Figure 2) explains the working principle of the idealistic OLED.

Negatively charged Electrons and positively charged holes are injected into the Lowest

Unoccupied Molecular Orbital (LUMO) and Highest Occupied Molecular Orbital (HOMO)

of the organic layers respectively, and move towards each other in the applied external field.

As they approach each other close enough, the recombination occurs due to attraction and

releases the energy as radiation (light).

This simple device geometry however

relies on the fact that exactly the same

number of electrons and holes are injected

into the organic layer to make sure that no

carrier passes the sample without meeting

a partner to recombine. Hence, the

injection barriers for electrons and holes

from the cathode and anode have to be

equal. Furthermore, the charge carrier

mobility for electrons and holes needs to

be equal for the recombination to take place in the centre of the organic layer.

OLED’s Production: Based on the type of organic component used, OLED’s are of two

types; made up of a) small molecules and b) macro molecules (polymers).

OLED’s based on small molecules usually manufactured via organic vapour phase

deposition or vacuum deposition; the organic molecules are evaporated and carried with an

inert gas followed by deposition on a substrate through very small nozzles held near the

substrate surface. This technique involves vacuum and temperatures higher than room

temperature which reflects in the higher production cost. However, highly uniform films can

be made and it is easy to fabricate complex multi-layer structures for higher efficiency.

Metal Cathode Emissive organic Layer Transparent Anode Layer

Substrate

Figure 2: Idealistic OLED under operation.

Metal

Cathode Metal

Anode

HOMO/Valence

band

LUMO/Conduction

band

Organic

layer

In the case of PLED’s, relatively small amount of power required for the amount of

light produced as PLED’s are quite efficient. Vacuum deposition is not a suitable method for

forming thin films of polymers. However, polymers can be processed in solution, and spin

coating is a common method of depositing thin polymer films. This method is more suited to

forming large area films than thermal evaporation. No vacuum is required, and the emissive

materials can also be applied on the substrate by a technique derived from commercial inkjet

printing. However, as the application of subsequent layers tends to dissolve those already

present, formation of multilayer structures is difficult with these methods.

Advantages of OLED Technology:

• Lightweight and flexible

• Ability to create a full spectrum color display

• Very bright with low energy requirements

• Wider viewing directions/angles

• Ability to create non-planar displays with light emitting polymer films

• Faster response times and high refresh rates